Numerical control including a variable gain loop controlling a pilot stepping motor controlling an electrohydraulic positioning motor

ABSTRACT

A pulse train converter converts a first command pulse train issued by command pulse means to a second command pulse train and supplies the second command pulse train to electrohydraulic pulse motor. The pulse train converter has a storage connected to the command pulse means for storing the difference in the number of pulses of the first and second command pulse trains and an oscillator connected to the storage for issuing the second command pulse train at a frequency proportional to the content of the storage. A comparer coupled between the command pulse means and the motor detects the difference between the number of pulses in the first command pulse train and the revolutions of the motor. A gain control connects the comparer to the pulse train converter in a manner whereby the ratio of the frequency of the second pulse train to the content of the storage is controlled so that the detected difference is in proportion to the frequency of the first command pulse train.

United States Patent [72] lnventors Seiuemon lnaba; 2,922,940 1/1960 Mergler 3l8/20.860 Klnryo Sill-1a, both of Kawasaki-511i, 3,030,054 4/1962 3 l 8/20.050 Japan 3,109,970 11/1963 3l8/20.430 X 211 A No. 878,059 3,241,015 3/1966 318120.430 X [22] Filed Nov. 19, 1969 3,320,502 /1967 3 l8/20.430 X Patented Sept. 14,197! 3,418,547 12/1968 Dudler 318120.860 {73] A5518: MM Primary Examiner-T. E. Lynch Anorneys-Curt M. Avery, Arthur E. Wilfond, Herbert L. [32] pnomy 1968 Lerner and Daniel J. Tick [33] Japan [31] 4345479 [54] NUMERICAL CONTROL INCLUDING A ABSTRACT: pulse train converter converts a first com- VARIABLE GAIN com-ROLLING A "LOT mand pulse train issued by comrnand pulse means to a second STEPHNG MOTOR Com-ROLLING AN command pulse tram and supplies the second command pulse ELECTROHYDRAUUC POSITIONING MOTOR train to electrohydraulic pulse motor. The pulse train con- 6 Cm 6 mm m verter has a storage connected to the command pulse means for storing the difference in the number of pulses of the first [52] US. 318/619, and second command puke trains and an oscillator connected 318/603 318/685 to the storage for issuing the second command pulse train at a Gosh 5/01 frequency proportional to the content of the storage. A com- Gosb 19/0 parer coupled between the command pulse means and the Field Search. 318/561, motor detects the diff between the number f l i 619 the first command pulse train and the revolutions of the motor. A gain control connects the comparer to the pulse train [56] Rehm Cm converter in a manner whe'reby the ratio of the frequency of UNITED STATES PATENTS the second pulse train to the content of the storage is con- 2,760,l3l 8/1956 Braunagel H 318/20.430 trolled so that the detected difference is in proportion to the 2,762,959 9/ 1956 Welch i '3 18/20.430 frequency of the first command pulse train.

PULSE TRAIN CONVERTER 101 ELECTROHYDRAULIC NUM RI A DECOISTRSL REGISTER PULSE MOTOR HYDRAULIC VICE I00 DIGITAL RESISTANC P s us rams in; E ii 5 3 n i i mr ne i I L 11 103 I I 82 S 7 I l 144 I g T L. 1 I I. I l I VARIABLE ROTARY PILOT I FRE u N i a :OSCILEA'FOR 1 O6 VALVE 2 I l A I I I 0 2300% I I 6 I- -12s L l 13/ 133 COMPARATOR 12a ,129 I I P ULSE GENERATOR VELOCITY ERROR 0F SERVOSYSTEM OUTPUT FREQfUENCY COMMAND PULSE 5000 FREQUENCY F 200 400 600 8001000 DIGITAL INFORMATION 5 R43 B2 ,CZn I E( g NUMERICAL CONTROL INCLUDING A VARIABLE GAIN The invention relates to an electrohydraulic pulse motor. More particularly, the invention relates to an error-correcting servosystem for an electrohydraulic pulse motor.

As known, an electrohydraulic pulse motor is a combination of an electric pulse motor, a rotary pilot valve and a hydraulic motor. When pulse trains of a specific frequency are supplied to the electric pulse motor there is an error or velocity error produced in the rotary pilot valve and the hydraulic motor rotates at a rate proportional to the error. When the load is constant, the error in the rotary pilot valve is proportional to the frequency of the pulse trains supplied to the electric pulse motor. When the load varies, the error produced in the rotary pilot valve varies accordingly. It is thus difficult to maintain the loop gain of the servomechanism constant. That is, it is difficult to maintain a constant ratio between the frequencies of the command pulse trains. When the servosystem is utilized in shaping an object by cutting its contour, if there are contour control variations in the loop gain, such variations will result in machine error. That is, the actual contour formed by cutting away material will differ from the desired contour.

The principal object of the invention is to provide a new and improved error-correcting servosystem for an electrohydraulic pulse motor.

An object of the present invention is to provide an errorcorrecting servosystem for an electrohydraulic pulse motor in which variations of loop gain are compensated for or prevented.

An object of the invention is to provide an error-correcting servosystem for electrohydraulic pulse motor which functions to correct for error in the system.

An object of the invention is to provide an error-correcting servosystem for an electrohydraulic pulse motor which functions with efficiency, effectiveness and reliability.

In accordance with the invention, an error-correcting servosystem for an electrohydraulic pulse motor comprises a pulse train converter for controlling the gain of the motor. Command pulse means supplies a command pulse train to the motor through the pulse train converter. The command pulse train has a frequency. A comparator is coupled between the command pulse means and the motor and determines the difference between the number of pulses in the command pulse train and the revolutions of the motor. Gain control means connects the comparator to the pulse train converter in a manner whereby error in the pulse train converter is controlled so that the difference is in proportion to the frequency of the command pulse train.

A first counter is connected between the command pulse means and the comparator for counting the command pulses. A reversible counter has an additive input connected to the command pulse means, a subtractive input coupled to the motor and an output connected to the comparator. A pulse generator is connected between the motor and the subtractive input of the reversible counter for providing a number of pulses proportional to the revolutions of the motor.

In accordance with the invention, a method of correcting error in an electrohydraulic pulse motor in a servosystem in which a command pulse train is supplied to the motor via a pulse train converter for controlling of the the motor comprises the steps of comparing the number of pulses in the command pulse train and the revolutions of the motor to determine the difference therebetween, and controlling the error in the pulse train converter in a manner whereby the difference is in proportion to the frequency of the command pulse train.

In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings, wherein;

FIG. 1 is a block diagram of an embodiment of the errorcorrecting servosystem of the invention for an electrohydraulic pulse motor;

FIG. 2 is a graphical presentation illustrating the relationship between the command pulse frequency and the velocity error;

FIG. 3 is a circuit diagram of the pulse train converter of FIG. 1;

FIG. 4 is a circuit diagram of the gain control circuit of FIG. 1;

FIG. 5 is a circuit diagram of a coupling circuit between the pulse train converter of FIG. 3 and the gain control circuit of FIG. 4; and

FIG. 6 is a graphical presentation of the frequency characteristics of the output pulse trains produced by the pulse train converter of FIG. 3.

In FIG. 1, a numerical control device comprises a pulse generator which produces command pulses. The command pulses are supplied to the servosystem. The command pulses from the numerical control device 100 are supplied to a pulse train converter 101 via lines 102 and 103.

The pulse train converter 101 comprises a register 104, a digital resistance or capacitance converter and a variable frequency oscillator 106. The line 103 is connected to an input of the register 104, which comprises a reversible counter. The output of the register 104 is connected to an input of the digital resistance converter 105. The output of the digital resistance converter 105 is connected to the input of the variable frequency oscillator 106. The output of the variable frequency oscillator 106 is connected to an output lead 107 and is also connected to an input of the register 104 via a feedback path 108. I

The pulse train converter 10] functions as a smoothing circuit. In its steady condition, the pulse train converter 101 provides a pulse train to the input line 107 having a frequency which is equal to that of the command pulses supplied to said pulse train converter. If the frequency of the command pulses suddenly varies, the pulse train converter 101 produces pulse trains having frequencies which vary smoothly. The pulse train converter 101 thus functions to prevent response error in the electrohydraulic pulse motor of the system.

The output line 107 from the variablefrequency oscillator 106 of the pulse train converter 101 is connected to an electrohydraulic pulse motor 109. The electrohydraulic pulse motor comprises an electric pulse motor 111. The input of the electric pulse motor 111 is the input of the electrohydraulic pulse motor 109. The output of the electric pulse motor 111 is coupled to the input of a rotary pilot valve 112. The output of the rotary pilot valve 1 12 is coupled to the input of a hydraulic motor 113. The output of the hydraulic motor 113 is coupled back to the rotary pilot valve 112 via a coupling 114. The hydraulic motor 113 has an output shaft 115, which is the output shaft of the electrohydraulic pulse motor 109. The output shaft 115 is coupled to a machine tool 116.

The command pulses of the pulse train converter are counted by the reversible counter or register 104. In the converter 10S and the variable frequency oscillator 106, the pulse trains having a frequency proportional to the count of the counter 104 are transferred to the output line 107. The command pulses supplied via the line 103 are additively counted by the reversible counter 104. The command pulses supplied via the line 108, to the subtractive input of the reversible counter 104, are subtractively counted by said counter, that is, each pulse supplied to the counter 104 via the line 108 results in the subtraction of one from the count of said counter.

The count el of the counter 104 thus includes the error of the pulse train converter 101. In the steady condition, that is, in the condition where the frequency F of the command pulses is equal to the frequency f of the pulses transferred via the line 107, the error el is proportional to the frequency F.

On the other hand, the error e2 of the electrohydraulic pulse motor 109 is produced in the rotary pilot valve 112 and is proportional to the input pulse frequency f of the electric pulse motor 111, when the load on said electrohydraulic pulse motor is constant. The steady condition error of the entire servomechanism is thus The object of the invention, which is to maintain the loop gain g constant, requires that the relationship between the frequency F of the command pulses and the steady condition error e of the entire servosystem be completely proportional as shown in FIG. 2. That is,

e=F/k wherein k is a proportionality constant.

When the load of the electrohydraulic pulse motor 109 varies, the error e2 of the rotary pilot valve 112 varies. Therefore, in accordance with our invention, the error el of the pulse train converter 101 should be changed in accordance with the amount of variation, so as to constantly maintain the total error e proportional to the command pulse frequency F. The ideal error es is proportional to the command pulse frequency F and is determined by a counter 117 in FIG. 1. The command pulses provided in the line 102 are supplied to an input of the counter 117 via said line, a line 118 and a gate 119.

The counter 117 counts the command pulses supplied thereto via the gate 119 during the period when pulses of specific duration are supplied to said gate via a line 121. The counter 117 has a reset input 122. The error es is thus the count of the counter 117. The error es is proportional to the frequency of the command pulses. A reversible counter 123 determines the difference between the number of command pulses and the number of actual revolutions of the electrohydraulic pulse motor 109, that is, the counter 108 determines the error of the entire servomechanism. The command pulses are fed to the additive input of the reversible counter 123 via the line 118 and a line 124. The output revolutions of the electrohydraulic pulse motor 109 are converted to electrical pulse generator 125 having an input coupled to the output of said electrohydraulic pulse motor via a coupling 126 and an output connected to the subtractive input of the reversible counter 123 via a line 127. The pulse generator 125 produces a number of pulses proportional to the revolutions of the electrohydraulic pulse motor 109.

The output of the counter 1 17 is connected to one input of a comparator 128 via a lead 129. The output of the reversible counter 123 is connected to another input of the comparator 128 via a line 131. The comparator 128 functions to compare the error e of the entire servomechanism with the ideal error es to determine which is greater. When a command pulse is supplied to the comparator 128 via a line 132, said comparator provides pulses in either of its output lines 133 and 134 in accordance with the results of such comparison. The output lines 133 and 134 of the comparator 128 are connected as the inputs to a gain control circuit 135. The output of the gain control circuit 135 is connected to the other input of the digital resistance converter 105 of the pulse train converter 101 via a line 136. The gain control circuit 135 functions to raise or increase the gain by one step when e is greater than es and to lower or decrease the gain by one step when e is less than es.

FIG. 3 is a circuit diagram of the pulse train converter 101 of FIG. 1. The pulse train converter 101 functions as a gain control circuit The command pulses from the numerical control device 100 of FIG. 1 are supplied to an input terminal 301 of FIG. 3. The command pulses supplied to the terminal 301 are supplied to the input of a register 302. The register 302 functions as the reversible counter 104 of FIG. 1. The register 302 records and stores the command pulses in binary notation and has a plurality of output lines 303 which are individually identified as L2, L2, L2 1.2"", L2", each output line being associated with a corresponding digit and each representing the binary numerical value 2, 2', 2 2" and 2", respectively.

The output lines 303 of the register 302 are connected to corresponding inputs of a digital resistance converter 304. The digital resistance converter 304 comprises a plurality of transistors 00, Q1, Qn1, On, which correspond to the output lines L2, L2, L2", 1.2" of the register 302. The base electrodes of the transistors Q0, Q1, Qn-l, Qn are connected to the output lines L2, L2, 1.2", L2", respectively,

via resistors R10, R11, R1n-1, R1n, respectively. The resistors R10, R11, Rln-l, Rln have equivalent resistance values.

The base electrodes of the transistors Q0, Q1, Qn-l, Qn are connected to a DC voltage source which provides -l2 volts, via resistors R20, R21, R2n-1. R2n, respectively, and a voltage point terminal 305. The resistors R20, R21, R2n-1, Rn have equivalent resistance values and are connected to the voltage terminal 305 via a line 306. The collector electrodes of the transistors Q0, Q1, Qn-I, Qn are connected to a DC voltage source which provides +16 volts via resistors R30, R31, R3n-1, R3n, respectively, a line 307 and a voltage terminal 308. The emitter electrodes of the transistors Q0, Q1, Qn-l, Qn are connected to a source 0 volts via a common line 309 and a voltage terminal 31 1.

A diode D0 and a resistor R40 are connected in series circuit arrangement between the collector electrode of the transistor Q0 and an output terminal 312 of the digital resistance converter 304. A diode D3 and a resistor R41 are connected in series circuit arrangement between the collector electrode of the transistor Q1 and the output terminal 312 of the converter 304. A diode Dn-l and a resistor R4n-1 are connected in series circuit arrangement between the collector electrode of the transistor Qn-l and the output terminal 312. A diode Dn and a resistor R4n are connected in series circuit arrangement between the collector electrode of the transistor On and the output terminal 312.

The output terminal 312 of the digital resistance converter 304 is connected to, and functions as the input terminal of, a unijunction transistor oscillator 313. The unijunction transistor oscillator 313 comprises a unijunction transistor Q5. The emitter electrode of the unijunction transistor O5 is directly connected to the terminal 312. The base electrode B2 of the unijunction transistor 05 is connected to the line 307 via a resistor R5. The base electrode B1 of the unijunction transistor Q5 is connected to the line 309 via a resistor R6. A capacitor C1 is connected in parallel between the emitter electrode E of the unijunction transistor Q5 and the line 309.

The base electrode B1 of the unijunction transistor O5 is connected to the base electrode of a transistor 06 of a wave shaping amplifier 314 via a line 315. The emitter electrode of the transistor O6 is directly connected to the line 306 and the collector electrode of said transistor is connected to the line 307 via a resistor R7. The collector electrode of the transistor Q6 is connected to the base electrode of a transistor Q7 via a resistor R8. The emitter electrode of the transistor O7 is directly connected to the line 306. The collector electrode of the transistor Q7 is connected to the line 307 via a resistor R9. An output terminal 316 is directly connected to the collector electrode of the transistor Q7.

In FIG. 5, 0 volts is the equivalent of the logical value 1 and +16 volts is the equivalent of the logical value 0. The resistance values of the resistors R40, R41, R4n-1, R4n and R30, R31, R3n-1, R3n, are

R30+R40=l0 times 2" kilohms R3 1+R41=l0 times 2" kilohms R3n-1 +R4n-l =10 times 2 kilohms R3n+R4n=l0 times 2 kilohms The resistance equations indicate that the total resistance for a pair of resistors associated with one transistor is one-half that of the next-preceding digit. That is, the combined resistance of the resistors R30 and R40, associated with the transistor Q0, which is connected to the lowest digit 2 of the register 302, is the largest. The combined resistance of the resistors R31 and R41, associated with the transistor Q1, is half the resistance value, and the remaining resistance values follow in the same manner.

The register 302 stores the digital information in binary notation and, in accordance with the binary value, supplies the 0 volts or +16 volts signal to the output terminal for each digit. If the value in the register is 5" in decimal notation the +0 voltage signal appears only at the output terminals L2 and L2 When the logical signal 0, or +16 volts, appears at all the output lines L2, L2. L2'", L2", all the transistors Q0, Q1,

Qn-l, Or: are in their conductive condition. When the logical value I, or volts, appears at some of the output lines L2", L2. L2'". L2", only those transistors connected to output lines at which 0 volts appear are in their nonconductive condi- 5 tion.

When the contents of the register 302 are zero, all the transistors are in their conductive condition, the collector potential of each transistor becomes zero volts, and since the diodes D0, D1. Dn are reverse biased, the resistors R40, R41, R4n-1, R4r| are disconnected from the circuit. When the register 302 has counted to its full capacity, all the transistors are in their nonconductive condition, the collector potential of each transistor increases, and the diodes D0, D1,

Dnl, Dn are forward biased. The resistors R40, R41, R41: are then connected in series with the resistors R30, R31,

R3n, The combined resistance R is derived as Consequently, if the resistance values of the resistance R30, R31: and R40, R41: are set for the aforedescribed value, the combined resistance of the line 307 and he output line of the terminal 312 will be a value which is inversely proportional to the binary value of the register 302.

The capacitor C1 is charged through the line including the terminal 312, the resistor circuit and said terminal. The emitter E potential of the unijunction transistor Q5 commences to increase, until the potential reaches the potential of the base electrode 82 of said unijunction transistor, which base electrode is 16 volts. When the base electrode B2 potential of the unijunction transistor Q5 reaches 16 volts, said unijunction transistor becomes conductive. The electrical charge stored in the apacitor C1 is then discharged via the base electrode Bl i f I inijunction transistor and the resistor R6.

When the potential of the emitter electrode E of the 40 unijunction transistor decreases to about 2 volts, said unijunction transistor becomes nonconductive and the capacitor C1 recharges. The cycle Tof the unijunction transistor oscillator 313 is defined as l=(2.3R)Clog Ill-1 Wherein R is the magnitude of the resistance, C is the magnitude of the capacitance and 1; is the parameter determined in accordance with the type of unijunction transistor. It is thus obvious that a cycle of the oscillator has little relation to the voltage and the temperature, and that considerably linear DAD conversion may be provided by digital variation of the resistance or capacitance magnitude.

Each time the unijunction transistor 05 becomes conductive, a positive voltage is provided in the output line 315. The voltage in the line 315 is applied to the amplifying transistor Q6 of the wave shaping amplifier 314. An output pulse is provided at the output terminal 316. The output pulse provided at the terminal 316 may also be applied to the subtracting input of the register 302 via a line 317.

FIG. 4 shows the gain control circuit 135 of FIG. 1. In FIG. 4, a register 40] has an additive input terminal 402 and a sub tractive input terminal 403. The additive input terminal 402 is connected to the line 134 of FIG. 1 and the subtractive input terminal 403 is connected to the line 133 of FIG. 1. The output lines 404, L2, L2, L2, 1.2", L2 of the register 401 for each digit of said register are connected in the same manner as in FIG. 3. The output lines 404 of the register 401 are therefore connected to corresponding base electrodes of a plurality of transistors O10, O11, Qln-l, Qln of a digital capacitance converter 405 via corresponding ones of a plurality of resistors R50, R51, RSn-l, R5n.

A diode D01 and a diode D02 are connected in series circuit arrangement between the emitter and collector electrodes of the transistor Q10. A diode D11 and a diode D12 are connected in series circuit arrangement between the emitter and collector electrodes of the transistor Q11. A diode Dln-l and a diode D2n1 are connected in series circuit arrangement between the emitter and collector electrodes of the transistor Q1n-1. A diode Dln and a diode D2n are connected in series circuit arrangement between the emitter and collector electrodes of the transistor Qln. A common point in the connection between the diodes D01 and D02 is connected to an output line 406 via a capacitor C20. A common point in the con nection between the diodes D11 and D12 is connected to the output line 406 via a capacitor C21. A common point in the connection between the diodes Dln-l and D2n-1 is connected to the output line 406 via a capacitor C2n-l. A common point in the connection between the diodes Dln and D2): is connected to the output line 406 via a capacitor C2n. The output line 406 connects the output of the digital capacitance converter 405 to the input of a unijunction transistor oscillator 407. The line 406 is connected to the emitter electrode E of a unijunction transistor Q8. The emitter electrode E of the unijunction transistor Q8 is connected to a line 408 via a resistor R10 and is connected to a line 409 via a capacitor C30.

The input digital information stored in the register 401 applies a potential of either 0 volts or 16 volts in accordance with the logical value 1 or 0 of each digit. These potentials are applied to the transistors Q10, Q11, Qln-l, Qln via the resistors R50, R51, R5n1, RSn. A transistor in conductive condition, to which 0 volts is applied, is switched to its nonconductive condition. A transistor in its nonconductive condition, to which 16 volts is applied, is switched to its conductive condition.

If the transistors Q10, Q11, Qln-l, Qln are nonconductive, the diodes D02, D12, D2n-1, D2n are reverse biased and become nonconductive and the corresponding capacitors C20, C21, C2n-l, C2n are thereby disconnected from the line 409. When the transistors Q10, Q11, Qln-l, Qln are in their conductive condition, the diodes D02 D12, D2n--1, D2n become conductive. Each of the corresponding capacitors C20, C21, C2n-1, C2n is then charged via the line 408, the resistor R10 and the line 406.

when the emitter potential of the unijunction transistor Q8 reaches the magnitude of the voltage of the voltage source, which is 16 volts, the electrical charge stored in the capacitors C20, C21, C2n-l, C2n is discharged via the base electrode B1 of said unijunction transistor, the resistor R12 and the diodes D01, D11, Dln-l, Dln. The combined capacitance C, which determines the oscillation cycle of the unijunction transistor oscillator 407, is defined as follows:

The following magnitudes are selected as the capacitance for each capacitor on the basis of the capacitance magnitude C20 for the capacitor C20:

C2l=C20 times 2 C2n1=C20 times 2"" C2n=C20 times 2" The oscillation cycle T of the unijunction transistor oscillator 407 is defined as T=2.3(R10)[C20+C21+... C2n1+C2n+C30 l-n] FIG. 5 shows the connection between the pulse train converter 101 and the gain control circuit of FIG. 1. More particularly, FIG. 5 shows the connection between the pulse train converter 101, the circuit of which is shown in FIG. 3, and the gain control circuit 135, the circuit of which is shown in FIG. 4. As shown in FIG. 5, the output terminal 312 of the digital resistance converter 304 of FIG. 3 and the output line 406 of the digital capacitance converter 405 of FIG. 4 are to be connected to the emitter electrode E of the unijunction transistor Q8.

FIG. 6 illustrates the frequency characteristics of the connection shown in FIG. 5. In FIG. 6, the abscissa indicates the accumulated value S of the digital information and the ordinate represents the output frequency f in pulses per second. The curve 1 of FIG. 6 illustrates the characteristic when the digital information stored in the register 401 is 1. The curves 2, 3, 4, S, 6, n of FIG. 6 show the frequency characteristic when the digital information stored in the register 401 is 2, 3, 4, n in decimal numbers.

llog [l/ As hereinbefore disclosed the servosystem of the invention automatically corrects errors in the pulse train converter, even when the load varies, thereby providing an ideal servomechanism.

While the invention has been described by means of a specific example and in a specific embodiment, we do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. An error-correcting servosystem for an electrohydraulic pulse motor, said servosystem comprising, command pulse means for issuing a first command pulse train; a pulse train converter for converting the first command pulse train to a second command pulse train and for supplying the second command pulse train to the electrohydraulic pulse motor, said pulse train converter having storage means connected to said command pulse means for storing the difference in the number of pulses of the first and second command pulse trains and oscillator means connected to said storage means for issuing said second command pulse train at a frequency proportional to the content of said storage means; comparing means coupled between the command pulse means and the motor for detecting the difference between the number of pulses in the first command pulse train and the revolutions of said motor; and gain control means connecting the comparing means to the pulse train converter in a manner whereby the ratio of the frequency of the second pulse train to the content of the storage means is controlled so that the detected difference is in proportion to the frequency of the first command pulse train,

2. An error-correcting servosystem as claimed in claim 1, further comprising a first counter connected between said command pulse means and said comparing means for counting the command pulses.

3. An error-correcting servosystem as claimed in claim 1, further comprising a reversible counter having an additive input connected to said command pulse means, a subtractive input coupled to said motor and an output connected to said comparing means.

4. An error-correcting servosystem as claimed in claim 1, further comprising a pulse generator coupled between said motor and said comparing means for providing a number of pulses proportional the revolutions of said motor.

5. An error-correcting servosystem as claimed in claim 1, further comprising a first counter connected between said command pulse means and said comparing means for counting the command pulses, a reversible counter having an additive input connected to said command pulse means, a subtractive input coupled to said motor and an output connected to said comparing means, and a pulse generator connected between said motor and the subtractive input of said reversible counter for providing a number of pulses proportional to the revolutions of said motor.

6. A method of error-correcting for an electrohydraulic pulse motor, comprising the steps of issuing a first command pulse train from a command pulse source; converting the first command pulse train to a second command pulse train and supplying the second pulse train to the electrohydraulic pulse motor; storing the difference in the number of pulses of the first and second command pulse trains; issuing the second command pulse train at a frequency proportional to the content of the stored difference, detecting the difference between the number of pulses in the first command pulse train and the revolutions of the motor; controlling the ratio of the frequency of the second pulse train to the content of the stored difference so that the detected difference is in proportion to the frequency of the first command pulse train. 

1. An error-correcting servosystem for an electrohydraulic pulse motor, said servosystem comprising, command pulse means for issuing a first command pulse train; a pulse train converter for converting the first command pulse train to a second command pulse train and for supplying the second command pulse train to the electrohydraulic pulse motor, said pulse train converter having storage means connected to said command pulse means for storing the difference in the number of pulses of the first and second command pulse trains and oscillator means connected to said storage means for issuing said second command pulse train at a frequency proportional to the content of said storage means; comparing means coupled between the command pulse means and the motor for detecting the difference between the number of pulses in the first command pulse train and the revolutions of said motor; and gain control means connecting the comparing means to the pulse train converter in a manner whereby the ratio of the frequency of the second pulse train to the content of the storage means is controlled so that the detected difference is in proportion to the frequency of the first command pulse train.
 2. An error-correcting servosystem as claimed in claim 1, further comprising a first counter connected between said command pulse means and said comparing means for counting the command pulses.
 3. An error-correcting servosystem as claimed in claim 1, further comprising a reversible counter having an additive input connected to said command pulse means, a subtractive input coupled to said motor and an output connected to said comparing means.
 4. An error-correcting servosystem as claimed in claim 1, further comprising a pulse generator coupled between said motor and said comparing means for providing a number of pulses proportional the revolutions of said motor.
 5. An error-correcting servosystem as claimed in claim 1, further comprising a first counter connected between said command pulse means and said comparing means for counting the command pulses, a reversible counter having an additive input connected to said command pulse means, a subtractive input coupled to said motor and an output connected to said comparing means, and a pulse generator connected between said motor and the subtractive input of said reversible counter for providing a number of pulses proportional to the revolutions of said motor.
 6. A method of error-correcting for an electrohydraulic pulse motor, comprising the steps of issuing a first command pulse train from a command pulse source; converting the first command pulse train to a second command pulse train and supplying the second pulse train to the electrohydraulic pulse motor; storing the difference in the number of pulses of the first and second command pulse trains; issuing the second command pulse train at a frequency proportional to the content of the stored differeNce, detecting the difference between the number of pulses in the first command pulse train and the revolutions of the motor; controlling the ratio of the frequency of the second pulse train to the content of the stored difference so that the detected difference is in proportion to the frequency of the first command pulse train. 